As shown in FIG. 1, a general CD is formed with a thickness of 1.2 mm and a diameter of 120 mm. The CD has a center hole of 15 mm in diameter, and a clamped region of 44 mm in diameter and formed around the center hole. The clamped region is fixed to a turntable of an optical disk apparatus by means of a clamper.
When the CD is properly placed on the optical disk apparatus, a recording layer, on which data are recorded in a pit pattern, is spaced approximately 1.2 mm from the surface of the CD opposite to an object lens of an optical pickup of the optical disk apparatus. The object lens of the optical pickup for the CD has a relatively small numeral aperture (NA). For example, the lens has an NA of 0.4-5.
As shown in FIG. 2, a general DVD is formed with a thickness of 1.2 mm and a diameter of 120 mm. The DVD has a center hole of 15 mm in diameter, and a clamped region of 44 mm in diameter and formed around the center hole. The clamped region is fixed to a turntable of an optical disk apparatus by means of a clamper.
When the DVD is properly placed on the optical disk apparatus, a recording layer, on which data are recorded in a pit pattern, is at a distance of approximately 0.6 mm from the surface of the DVD opposite to an object lens of an optical pickup of the optical disk apparatus. The object lens of the optical pickup for the DVD has a relatively large numeral aperture (NA). For example, the lens has an NA of 0.6.
As shown in FIG. 3, a newly commercialized HD-DVD is formed with a thickness of 1.2 mm and a diameter of 120 mm. The HD-DVD has a center hole of 15 mm in diameter, and a clamped region of 44 mm in diameter formed around the center hole. The clamped region is fixed to a turntable of an optical disk apparatus by means of a clamper. When the HD-DVD is properly placed on the optical disk apparatus, a recording layer, on which data are recorded in a pit pattern, is spaced approximately 0.1 mm from the surface of the HD-DVD opposite to an object lens of an optical pickup of the optical disk apparatus.
The object lens of the optical pickup for the HD-DVD has the largest numeral aperture (NA). For example, the lens has an NA of 0.85. The optical pickup for the HD-DVD uses a laser beam having a short wavelength to reproduce data recorded in high density or record data in high density, as compared to the optical pickup for the CD or the DVD.
The object lens of the optical pickup for the HD-DVD is closer to the recording layer of the HD-DVD than that for the CD or the DVD. Furthermore, the object lens of the optical pickup for the HD-DVD has the largest numeral aperture, and the optical pickup for the HD-DVD uses the laser beam having a shorter wavelength than that for the CD or the DVD. Consequently, an increased amount of light can be focused on the recording layer formed in the pit pattern using a small beam spot, and the light transmittance distance of the laser beam having the short wavelength can be shortened.
When a HD-DVD 10 is properly placed on a turntable 11 of the optical disk apparatus, as shown in FIG. 4, the HD-DVD 10 is rotated at high speed by a servo operation of a spindle motor 12, a motor driving unit 13, and a servo controller 15. While the HD-DVD 10 is rotated, the object lens OL of an optical pickup 14 of the optical disk apparatus focus on the HD-DVD 10 on the basis of a focusing servo operation by which the object lens OL of the optical pickup 14 is vertically moved within the range of an operating distance OD so that reading high density data recorded in the pit pattern or recording high density data in the pit pattern is normally carried out.
When the HD-DVD 10 is placed upside down on the turntable 11 of the optical disk apparatus, or one of other different kinds of optical disks is placed on the turntable 11 of the optical disk apparatus, for example, when a general CD 20 is placed on the turntable 11 of the optical disk apparatus, as shown in FIG. 5, the CD 20 is also rotated at high speed by a servo operation of the spindle motor 12, the motor driving unit 13, and the servo controller 15. In this case, the recording layer of the CD placed on the turntable 11 of the optical disk apparatus is at a distance of 1.1 mm more from the surface of the CD opposite to the object lens OL of the optical pickup 14 of the optical disk apparatus, as compared to the recording layer of the HD-DVD.
Under the aforesaid condition, the object lens OL of the optical pickup 14 of the optical disk apparatus cannot focus on the CD 20 by a normal focusing operation for moving vertically the object lens OL. For this reason, the object lens OL of the optical pickup 14 of the optical disk apparatus is upwardly moved toward the recording layer of the CD 20 until it moves the maximum operating distance OD_Max under the control of the servo controller 15 for controlling the focusing operation. Consequently, the object lens OL of the optical pickup 14 of the optical disk apparatus comes into collision with the CD 20, which may cause the CD 20 or the object lens OL of the optical pickup 14 to be damaged, or otherwise cause the servo operation to have fatal errors.